THE ENGLISH SPAHEOW 419 



were present even in this oldest known Ijird. In the Creta- 

 ceous rock deposits of the Great Plains there have been found 

 fossil birds with teeth set in sockets or grooves, preciseh' as 

 they are to-day in reptiles (Fig. 388). These remains show 

 us in the clearest manner that birds have been derivefl from 

 reptiles or both from a common source. Indeed, the two 

 groups are closely related anatomically, and are often united 

 under the name Sauropsida, or lizard-like animals. 



